14 research outputs found
β‑Diketiminato Nickel Imides in Catalytic Nitrene Transfer to Isocyanides
The β-diketiminato nickelÂ(I)
species [Me<sub>3</sub>NN]ÂNiÂ(2-picoline)
(<b>1</b>) serves as an efficient catalyst for carbodiimide
(RNCNR′) formation in the reactions of a range
of organoazides N<sub>3</sub>R with isocyanides R′NC. [Me<sub>3</sub>NN]ÂNiÂ(CNR)<sub>2</sub> (R = <sup>t</sup>Bu, Ar (Ar = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)) species provide carbodiimides
RNCNAr′ upon reaction with Ar′N<sub>3</sub> (Ar′ = 3,5-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>). Nitrene transfer takes place via the intermediacy of nickel imides.
Reaction of [Me<sub><i>x</i></sub>NN]ÂNiÂ(2-picoline) (<i>x</i> = 2 or 3) with Ar′N<sub>3</sub> gives the new dinickel
imides {[Me<sub><i>x</i></sub>NN]ÂNi}<sub>2</sub>(μ-NAr′)
(<b>4</b> (<i>x</i> = 3) and <b>5</b> (<i>x</i> = 2)) as deep purple, diamagnetic substances. The X-ray
structure of {[Me<sub>2</sub>NN]ÂNi}<sub>2</sub>(μ-NAr′)
(<b>5</b>) features short Ni–N<sub>imide</sub> distances
of 1.747(2) and 1.755(2) Å along with a short Ni–Ni distance
of 2.7210(3) Ã…. These dinickel imides <b>4</b> and <b>5</b> react stoichiometrically with <sup>t</sup>BuNC to provide
the corresponding carbodiimides <sup>t</sup>BuNCNAr′
in good yield. Azide transfer takes place upon reaction of <b>1</b> with TMS-N<sub>3</sub> to give the square planar nickelÂ(II) azide
[Me<sub>3</sub>NN]ÂNiÂ(N<sub>3</sub>)Â(2-picoline) (<b>7</b>).
Stoichiometric reaction of dinickel dicarbonyl {[Me<sub>3</sub>NN]ÂNi}<sub>2</sub>(μ-CO)<sub>2</sub> with organoazides such as Ar′N<sub>3</sub> is sluggish, indicating that <b>1</b> is not an efficient
catalyst for nitrene transfer from organoazides to CO to form isocyanates
RNî—»Cî—»O
Electrocatalytic Hydrogen Production by [Ni(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup>: Removing the Distinction Between Endo- and Exo-Protonation Sites
A new NiÂ(II) complex, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>H]<sup>3+</sup> (7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane),
has been synthesized, and its electrochemical properties have been
reported. The 7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> ligand features
an NH, ensuring properly positioned protonated amine groups (N–H<sup>+</sup>) for electrocatalysis, regardless of whether protonation
occurs exo or endo to the metal center. The compound is an electrocatalyst
for H<sub>2</sub> production in the presence of organic acids (p<i>K</i><sub>a</sub> range 10–13 in CH<sub>3</sub>CN), with
turnover frequencies ranging from 160 to 780 s<sup>–1</sup> at overpotentials between 320 and 470 mV, as measured at the potential
of the catalytic wave. In stark contrast to [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub> = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane)
and other [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup>)<sub>2</sub>]<sup>2+</sup> complexes, catalytic turnover frequencies for H<sub>2</sub> production by [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> do not show catalytic rate enhancement upon
the addition of H<sub>2</sub>O. This finding supports the assertion
that [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> eliminates the distinction between the endo- and exo-protonation
isomers
Electrocatalytic Hydrogen Production by [Ni(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup>: Removing the Distinction Between Endo- and Exo-Protonation Sites
A new NiÂ(II) complex, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>H]<sup>3+</sup> (7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane),
has been synthesized, and its electrochemical properties have been
reported. The 7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> ligand features
an NH, ensuring properly positioned protonated amine groups (N–H<sup>+</sup>) for electrocatalysis, regardless of whether protonation
occurs exo or endo to the metal center. The compound is an electrocatalyst
for H<sub>2</sub> production in the presence of organic acids (p<i>K</i><sub>a</sub> range 10–13 in CH<sub>3</sub>CN), with
turnover frequencies ranging from 160 to 780 s<sup>–1</sup> at overpotentials between 320 and 470 mV, as measured at the potential
of the catalytic wave. In stark contrast to [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub> = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane)
and other [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup>)<sub>2</sub>]<sup>2+</sup> complexes, catalytic turnover frequencies for H<sub>2</sub> production by [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> do not show catalytic rate enhancement upon
the addition of H<sub>2</sub>O. This finding supports the assertion
that [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> eliminates the distinction between the endo- and exo-protonation
isomers
C–H Functionalization Reactivity of a Nickel–Imide
We report bifunctional reactivity of the β-diketiminato
NiÂ(III)–imide
[Me<sub>3</sub>NN]ÂNiî—»NAd (<b>1</b>), which undergoes
H-atom abstraction (HAA) reactions with benzylic substrates R–H
(indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me<sub>3</sub>NN]ÂNi–NHAd (<b>2</b>) for the resulting hydrocarbyl
radical R<sup>•</sup> to give the nickel–amide [Me<sub>3</sub>NN]ÂNi–NÂ(CHMePh)ÂAd (<b>3</b>) (R–H = ethylbenzene)
or aminoalkyl tautomer [Me<sub>3</sub>NN]ÂNiÂ(η<sup>2</sup>-CHÂ(Ph)ÂNHAd)
(<b>4</b>) (R–H = toluene). A significant amount of functionalized
amine R–NHAd is observed in the reaction of <b>1</b> with
indane along with the dinickel imide {[Me<sub>3</sub>NN]ÂNi}<sub>2</sub>(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to
rate-limiting HAA from R–H by <b>1</b> to give R<sup>•</sup>, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus,
these studies illustrate a fundamental competition possible in C–H
amination systems that proceed via a HAA/radical rebound mechanism
C–H Functionalization Reactivity of a Nickel–Imide
We report bifunctional reactivity of the β-diketiminato
NiÂ(III)–imide
[Me<sub>3</sub>NN]ÂNiî—»NAd (<b>1</b>), which undergoes
H-atom abstraction (HAA) reactions with benzylic substrates R–H
(indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me<sub>3</sub>NN]ÂNi–NHAd (<b>2</b>) for the resulting hydrocarbyl
radical R<sup>•</sup> to give the nickel–amide [Me<sub>3</sub>NN]ÂNi–NÂ(CHMePh)ÂAd (<b>3</b>) (R–H = ethylbenzene)
or aminoalkyl tautomer [Me<sub>3</sub>NN]ÂNiÂ(η<sup>2</sup>-CHÂ(Ph)ÂNHAd)
(<b>4</b>) (R–H = toluene). A significant amount of functionalized
amine R–NHAd is observed in the reaction of <b>1</b> with
indane along with the dinickel imide {[Me<sub>3</sub>NN]ÂNi}<sub>2</sub>(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to
rate-limiting HAA from R–H by <b>1</b> to give R<sup>•</sup>, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus,
these studies illustrate a fundamental competition possible in C–H
amination systems that proceed via a HAA/radical rebound mechanism
C–H Functionalization Reactivity of a Nickel–Imide
We report bifunctional reactivity of the β-diketiminato
NiÂ(III)–imide
[Me<sub>3</sub>NN]ÂNiî—»NAd (<b>1</b>), which undergoes
H-atom abstraction (HAA) reactions with benzylic substrates R–H
(indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me<sub>3</sub>NN]ÂNi–NHAd (<b>2</b>) for the resulting hydrocarbyl
radical R<sup>•</sup> to give the nickel–amide [Me<sub>3</sub>NN]ÂNi–NÂ(CHMePh)ÂAd (<b>3</b>) (R–H = ethylbenzene)
or aminoalkyl tautomer [Me<sub>3</sub>NN]ÂNiÂ(η<sup>2</sup>-CHÂ(Ph)ÂNHAd)
(<b>4</b>) (R–H = toluene). A significant amount of functionalized
amine R–NHAd is observed in the reaction of <b>1</b> with
indane along with the dinickel imide {[Me<sub>3</sub>NN]ÂNi}<sub>2</sub>(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to
rate-limiting HAA from R–H by <b>1</b> to give R<sup>•</sup>, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus,
these studies illustrate a fundamental competition possible in C–H
amination systems that proceed via a HAA/radical rebound mechanism
Hydrogen Production Using Nickel Electrocatalysts with Pendant Amines: Ligand Effects on Rates and Overpotentials
A Ni-based electrocatalyst for H<sub>2</sub> production, [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]Â(BF<sub>4</sub>)<sub>2</sub>,
featuring eight-membered cyclic
diphosphine ligands incorporating a single amine base, 1-<i>para</i>-bromophenyl-3,7-triphenyl-1-aza-3,7-diphosphacycloheptane (8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>) has
been synthesized and characterized. X-ray diffraction studies reveal
that the cation of [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>(CH<sub>3</sub>CN)]Â(BF<sub>4</sub>)<sub>2</sub> has a distorted trigonal bipyramidal geometry. In CH<sub>3</sub>CN, [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> is an electrocatalyst
for reduction of protons, and it has a maximum turnover frequency
for H<sub>2</sub> production of 800 s<sup>–1</sup> with a 700
mV overpotential (at <i>E</i><sub>cat/2</sub>) when using
[(DMF)ÂH]ÂOTf as the acid. Addition of H<sub>2</sub>O to acidic CH<sub>3</sub>CN solutions of [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> results in
an increase in the turnover frequency for H<sub>2</sub> production
to a maximum of 3300 s<sup>–1</sup> with an overpotential of
760 mV at <i>E</i><sub>cat/2</sub>. Computational studies
carried out on [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> indicate the observed
catalytic rate is limited by formation of nonproductive protonated
isomers, diverting active catalyst from the catalytic cycle. The results
of this research show that proton delivery from the exogenous acid
to the correct position on the proton relay of the metal complex is
essential for fast H<sub>2</sub> production
High Catalytic Rates for Hydrogen Production Using Nickel Electrocatalysts with Seven-Membered Cyclic Diphosphine Ligands Containing One Pendant Amine
A series of Ni-based electrocatalysts, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]Â(BF<sub>4</sub>)<sub>2</sub>, featuring seven-membered cyclic diphosphine ligands incorporating
a single amine base, 1-<i>para</i>-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane
(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>, where X = OMe, Me,
Br, Cl, or CF<sub>3</sub>), have been synthesized and characterized.
X-ray diffraction studies have established that the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> complexes
have a square planar geometry, with bonds to four phosphorus atoms
of the two bidentate diphosphine ligands. Each of the complexes is
an efficient electrocatalyst for hydrogen production at the potential
of the NiÂ(II/I) couple, with turnover frequencies ranging from 2400
to 27 000 s<sup>–1</sup> with [(DMF)ÂH]<sup>+</sup> in
acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis,
giving turnover frequencies ranging from 4100 to 96 000 s<sup>–1</sup>. Computational studies carried out on the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> family
indicate the catalytic rates reach a maximum when the electron-donating
character of X results in the p<i>K</i><sub>a</sub> of the
NiÂ(I) protonated pendant amine matching that of the acid used for
proton delivery. Additionally, the fast catalytic rates for hydrogen
production by the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> family relative to the analogous [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family are attributed to preferred formation of endo protonated
isomers with respect to the metal center in the former, which is essential
to attain suitable proximity to the reduced metal center to generate
H<sub>2</sub>. The results of this work highlight the importance of
precise p<i>K</i><sub>a</sub> matching with the acid for
proton delivery to obtain optimal rates of catalysis
High Catalytic Rates for Hydrogen Production Using Nickel Electrocatalysts with Seven-Membered Cyclic Diphosphine Ligands Containing One Pendant Amine
A series of Ni-based electrocatalysts, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]Â(BF<sub>4</sub>)<sub>2</sub>, featuring seven-membered cyclic diphosphine ligands incorporating
a single amine base, 1-<i>para</i>-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane
(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>, where X = OMe, Me,
Br, Cl, or CF<sub>3</sub>), have been synthesized and characterized.
X-ray diffraction studies have established that the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> complexes
have a square planar geometry, with bonds to four phosphorus atoms
of the two bidentate diphosphine ligands. Each of the complexes is
an efficient electrocatalyst for hydrogen production at the potential
of the NiÂ(II/I) couple, with turnover frequencies ranging from 2400
to 27 000 s<sup>–1</sup> with [(DMF)ÂH]<sup>+</sup> in
acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis,
giving turnover frequencies ranging from 4100 to 96 000 s<sup>–1</sup>. Computational studies carried out on the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> family
indicate the catalytic rates reach a maximum when the electron-donating
character of X results in the p<i>K</i><sub>a</sub> of the
NiÂ(I) protonated pendant amine matching that of the acid used for
proton delivery. Additionally, the fast catalytic rates for hydrogen
production by the [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup>)<sub>2</sub>]<sup>2+</sup> family relative to the analogous [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>C6H4X</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family are attributed to preferred formation of endo protonated
isomers with respect to the metal center in the former, which is essential
to attain suitable proximity to the reduced metal center to generate
H<sub>2</sub>. The results of this work highlight the importance of
precise p<i>K</i><sub>a</sub> matching with the acid for
proton delivery to obtain optimal rates of catalysis
No effects of fingolimod on spinal cord glutamate transporter protein levels during EAE.
<p>(A-D) Western Blot analyses of spinal cord homogenates relative to GAPDH (A) or beta actin (C) with the respective densitometry relative to the housekeeping gene (B,D). At the maximum of EAE, there was a decrease for SLC1A2 (A,B) and SLC1A3 protein levels (C,D) as compared to naïve mice which was restored after fingolimod treatment (3 mg/kg once daily). One out of three experiments is shown, n = 3 per group. (E-H) Laser scanning microscopy of spinal cord cross sections after staining for GFAP (red) and SLC1A2 (green, E,F) or SLC1A3 (green, G,H). Arrows indicate double labelled profiles. Representative images of spinal cord cross sections are shown. Bar = 50 μm.</p